1. Field of the Invention
The invention relates to an image forming apparatus and, more particularly, to an image forming apparatus having a guide member for specifying a conveying path of a transfer material which is led between an image bearing member and a transfer apparatus.
2. Related Background Art
A conventional image forming apparatus has an image bearing member on which an electrostatic latent image is formed in accordance with image information, developing means for visualizing the electrostatic latent image formed on the image bearing member, and a transfer apparatus for transferring a toner image on the image bearing member onto a transfer material.
In order to transfer the toner image held on the image bearing member onto the transfer material, it is necessary to pass the transfer material between the image bearing member and the transfer apparatus at a proper timing or always at a proper insertion angle or the like. For this purpose, a guide member which is electrically connected to the ground and specifies a passage is provided for the transfer material on the upstream side relative to the image bearing member in the conveying direction of the transfer material.
However, a transfer current under an environment of a high humidity, namely, charges applied by a transfer apparatus onto the transfer material whose resistance is reduced by being subjected to a moisture flow to the guide member connected to the ground and a transfer efficiency shown by a toner amount on the transfer material to a toner amount on the image bearing member deteriorates. Therefore, a construction for charging the guide member to an electric potential having the same polarity as that of the discharge charges of the transfer current, namely, a polarity opposite to that of toner is considered. As a method for realizing such a construction, there are known the following methods such as a method shown in Japanese Patent Publication No. 61-32667 wherein a bias power source is used as a guide member; a method shown in Japanese Patent Publication No. 4-58031 wherein a varistor serving as a constant voltage element is connected to a guide member and an electric potential of the guide member is charged to about 600V by receiving a transfer current; a method wherein in order to merely obtain a guide member of an electric potential having a polarity opposite to that of toner, the guide member is set into an electrically insulating state and a voltage is stepped up by a transfer current; and the like.
Further, in order to prevent a defective transfer, there is known also a technique such that an insulating state is selected under only an environment of a high humidity and a guide member is connected to the ground under another environment.
The method of setting the electric potential of the guide member in the conventional image forming apparatus, however, has the following problems.
First, according to the method of connecting the guide member to the ground, the transfer efficiency under the high humidity environment deteriorates as mentioned above and in the connection to the ground under the high humidity environment, there is a case where the transfer efficiency is below 60%, so that it is not efficient. On the other hand, according to the method of charging the guide member to the electric potential of the polarity opposite to that of the toner in order to maintain the transfer efficiency, another problem occurs such that the transfer material becomes dirty. In other words, when the guide member is charged to the electric potential of the polarity opposite to that of the toner, an operation for electrically attracting the toner to the transfer material acts and the transfer material becomes dirty.
According to the method of connecting the varistor to the guide member and charging the electric potential of the guide member to about 600 V by receiving the transfer current, under a low humidity environment in which a charged charge amount of toner increases, an electric adsorbing force is enhanced and the toner is deposited onto the guide member, so that a problem occurs such that the toner is adhered to the transfer material and the transfer material becomes dirty.
Further, a contamination of the transfer material, namely, a contamination of the guide member is more remarkable as the electric potential to be applied to the guide member of the polarity opposite to that of the toner becomes higher. When the guide member is set into the electrically insulating state, the electric potential during the continuous copying operation reaches 1000V or higher. Even in the high humidity environment as well as such a low humidity environment, the toner is likely to be adhered to the guide member which was charged to a high voltage. Since the guide member is electrically insulated, assuming that it is charged to 1000V or more, when a problem such as a jam or the like occurs, there is a case where the user directly touches the jammed transfer material and removes it. Therefore, it is prefered.
Consequently, even if the guide member is made insulative only under the high humidity environment and is connected to the ground under another environment, the contamination of the guide member occurs under the high humidity environment. In case of switching the guide member to the insulating state in which there is no electrical connection and the state in which the guide member is connected to the ground, the charges accumulated in the insulating state are hard to escape even after the elapse of time. When the guide member is switched to the ground connecting state, a spark discharge occurs and electric noises are generated. Therefore, it is necessary to take a countermeasure against the noises for the other electric parts. A problem such as an increase in burden on costs occurs.